n. sn>1. [ Gr. 'ana` back, way from + fy`laxis security, protection. ] 1. (Med.) Hypersensitivity (to a foreign antigen) resulting from prior contact with the same or a related substance. [ WordNet 1.5 + AS ] 2. (Med.) An immediate but transient systemic reaction to an antigenic substance to which a person has become hypersensitized; it is characterized by the contraction of smooth muscle and capillary dilation. [ Stedman ] [ PJC + AS ] The cascade of events is triggered by binding of the foreign antigen to immunoglobulin E and release of mediators such as histamine, bradykinin, serotonin, and slow-reacting substance from mast cells. It involves more than one organ system. The most commonly involved organs are skin, cardiovascular, respiratory and gastrointestinal systems. Anaphylaxis is manifested by a range of signs and symptoms including pruritus, urticaria, itching, dyspnea, abdominal pain and cardiovascular collapse (shock). A severe form of the reaction is called anaphylactic shock. [ Stedman ] [ PJC + AS ] 3. (Med) Anaphylactic shock. [ AS ] | 